Interpretive Summary: 1) Rationale: The goal here was to determine where calcium is localized during its role in the sensing of low oxygen conditions in maize during flooding. 2) Accomplishments: It was determined that calcium appears to have an important role in the early detection of low oxygen stress in maize seedlings during flooding conditions. A cell organelle (mitochondria) appears to be the site that receives the low oxygen signal and releases calcium, so that the signal can be transferred to activate genes. 3) Significance: The understanding of calcium's role in a plant's detection of low oxygen-stress conditions will allow for a greater understanding of how these conditions cause a plant to alter protein synthesis in response to flooding and may allow for effective methods to produce crop plants that are tolerant to flooding.

Technical Abstract:
Anoxia induces a rapid elevation of the cytosolic calcium ([Ca2+]cyt) in maize cells. The [Ca2+]cyt elevation is brought about by a release of the ion from intracellular stores. Moreover, the anoxic Ca2+ release is important for gene activation and survival in oxygen-deprived maize seedlings and cells. In this study, we examined the contribution of mitochondrial Ca2+ to the anoxic [Ca2+]cyt rise in maize cells. Imaging of intramitochondrial calcium levels showed that a majority of mitochondria released their calcium in response to anoxia and took up calcium upon reoxygenation. We further investigated if the mitochondrial Ca2+ release contributes to rise in cytosolic Ca2+, under anoxia. Analysis of spatial association between anoxic [Ca2+]cyt changes and the distribution of mitochondria and other intracellular calcium stores revealed that the largest [Ca2+]cyt increases occurred close to mitochondria and away from the tonoplast. In addition, carbonylcyanide p-trifluoro methoxyphenyl hydrazone treatment depolarized mitochondria and caused a mild elevation of [Ca2+]cyt in aerobic conditions but prevented [Ca2+]cyt increase in response to a subsequent anoxic pulse. These results suggest that mitochondria play an important role in the anoxic elevation of [Ca2+]cyt and participate in the signaling of O2 deprivation.